Although fluid valves have been developed and improved over many decades, no one type of valve is appropriate for all types of service. A variety of valve seats, flow obstruction geometries, actuators, seals and valve bodies have evolved to match a variety of fluid control needs. The type of need that this invention was specifically directed to meet is the need to control leakage in the mechanical operating means when corrosive, poisonous or high pressure fluids are present. However. the invention also has other applications where very high reliability, safety and integrity of operation is required.
All valves require seals to prevent external leakage. Normally, this requires sliding and/or rotating seals at the valve stem. This dynamic seal, if it does not operate properly, is the primary cause of external fluid leakage. The valve stem is normally the only non-fluid opening in the valve body, allowing actuation and control of the fluid obstruction attached to the valve stem. If the valve is needed for high pressure service, this body opening and seal becomes critical. Seal forces must be increased to withstand the high fluid pressures. At the same time, seal forces cannot be increased too much as this precludes a restricting reasonable torque from actuating the valve. The high pressure also creates high forces at the valve seat, further increasing needed actuating torque. Torque requirements become critical when the valve is located in areas where even torque limitations occur, cannot be applied, such as in restricted access areas or underwater or during emergencies.
In addition to these design constraints and tradeoffs, seal wear is accelerated at higher seal loadings, leading to premature failure and/or increased torque requirements. Increased friction can actually cause seizure of "freezing" of the valve. Over torque applied to free these valves can further damage the valve.
Cost of high pressure valves is also impacted because of the precision manufacture, machining and assembly required for these seals and components. Very smooth surface finish on sealing surfaces, a high degree of quality control. precise dimensional and out of roundness tolerances and limited axial throw is required, increasing cost over and above the direct increase in strength requirements of the high pressure.
Reliability of high pressure valve is also more critical than low pressure valves. Failure of a low pressure valve to stop flow may be addressed by several alternatives, i.e.: opening the line and capping the source, pinch off of a line, or on-line repair of the valve (with fluid loss). Failure of a very high pressure valve to close or leakage at the stem seal is much more serious. Opening the line and capping the source may not be possible due to excessive forces required. Pinch off of a line is only possible with deformable (thin wall) tubing or piping. If the fluid is toxic or polluting, very high valve reliability becomes a critical design requirement rather than a design concern. Some intregral containment of dangerous fluids is achieved with the use of flexible diaphragms or bellow replacing the seal or stuffing box. However, these means reduce the motion of the stem controlling the valve and have life limited by metal fatigue.
Prior art attempting to meet this need has led to (1) sealed valves which fully enclose the valve stem actuator and valve, (2) a change in the type of valve to be used which does not require external control/actuation or can be simply controlled, (3) over powering actuators for high torque loads, or (4) use of flexible metal bellows.
An example of the enclosure approach is an electric motor or solenoid actuator which is exposed to the control fluid at the high pressures. Control is accomplished by electrical signals from outside the pressure cavity. Electrical leads do not rotate or move transversely and static seals can be used. The difficulty with this approach is that the actuator may not be compatible with the control fluid. Even with compatible fluids, rapid changes in fluid pressure and temperature may cause the exposed actuator to fail. Because the pressure vessel is now enclosing the entire actuator as well as valve, the weight of the valve goes up dramatically. Maintenance and off line repair costs are also made more difficult. Reliability of external electric power source can also be a problem. Finally, the electrical leads are frequently subject to damage during installation or maintenance of the heavy valve.
An example of the change in valve type is a poppet or check valve. A check valve requires no external control, but is capable of stopping flow in only one direction, not proportional control or dead tight shutoff. A poppet valve can be remotely actuated (outside the pressure enclosure) by solenoids or magnets. However, it is also limited in proportional fluid control ability and dead tight shutoff.
An example of the overpowering approach is geared electrical motors or hydraulic actuators. Very high seal loads and torque are accepted and overpowered by large actuators. However, this approach does not cure excessive cost, seal wear and other problems previously discussed while creating a very large and heavy valve.
All of these prior art approaches have limited application. All which allow proportional control involve permanent actuators adding weight and cost. Those lighter in weight do not have proportional control ability. None allow normal proportional operation without structural attachment of the actuator, exposing the actuator and valve to tampering or vandalism. Bellows and diaphragms are not usable at high pressures and have limited life.
Magnetic couplings have been used in rotary drive applications for many years. Until recently magnetic couplings were limited to low torque operation of small specialty pumps or other similar applications. Development of rare earth magnetic couplings now allows higher torque levels. They contribute complete fluid integrity while concurrently reducing the torque.